Andrew Jacobs  of The New York Times reports health experts in Brazil have a word of advice for the Olympic marathon swimmers, sailors and windsurfers competing in Rio de Janeiro’s picture-postcard waters next month: Keep your mouth closed.

Despite the government’s promises seven years ago to stem the waste that fouls Rio’s expansive Guanabara Bay and the city’s fabled ocean beaches, officials acknowledge that their efforts to treat raw sewage and scoop up household garbage have fallen far short.

In fact, environmentalists and scientists say Rio’s waters are much more contaminated than previously thought.

Recent tests by government and independent scientists revealed a veritable petri dish of pathogens in many of the city’s waters, from rotaviruses that can cause diarrhea and vomiting to drug-resistant “super bacteria” that can be fatal to people with weakened immune systems.

Researchers at the Federal University of Rio also found serious contamination at the upscale beaches of Ipanema and Leblon, where many of the half-million Olympic spectators are expected to frolic between sporting events.

“Foreign athletes will literally be swimming in human crap, and they risk getting sick from all those microorganisms,” said Dr. Daniel Becker, a local pediatrician who works in poor neighborhoods. “It’s sad, but also worrisome.”

Government officials and the International Olympic Committee acknowledge that, in many places, the city’s waters are filthy. But they say the areas where athletes will compete — like the waters off Copacabana Beach, where swimmers will race — meet World Health Organization safety standards.

Even some venues with higher levels of human waste, like Guanabara Bay, present only minimal risk because athletes sailing or windsurfing in them will have limited contact with potential contamination, they add.

Still, Olympic officials concede that their efforts have not addressed a fundamental problem: Much of the sewage and trash produced by the region’s 12 million inhabitants continues to flow untreated into Rio’s waters.

“Our biggest plague, our biggest environmental problem, is basic sanitation,” said Andrea Correa, the top environmental official in the state of Rio de Janeiro. “The Olympics has woken people up to the problem.”

“A giant pipe running from downtown churns human waste into the marina [on Guanabara Bay] at certain times each day. Rats roam around in the waste. The stench makes uninitiated visitors feel like vomiting or fainting,” USA Today’s Martin Rogers reported Tuesday, less than two weeks before the Games kick off on Aug. 5.